One of the machines I built a year or so ago randomly boots up in the middle of the night. When in sleep mode, the fans will ramp up to max and back down at random intervals. I've reinstalled windows. Turned off wake on lan and all that junk the the BIOS. I updated to the latest bios. I turned off wake timers. I can't think of anything else to try.
Also by boot in the middle of the night, I don't mean from sleep, I mean from a full shutdown.